This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. P2001-294957, filed on Sep. 26, 2001; the entire contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to an apparatus for optical modulation and a method of optical modulation.
2. Description of the Related Art
Current high-speed/high-density wavelength division multiplexing optical transmission systems provide transmission equipment and intermediate repeaters with optical fiber amplifiers for the compensation of optical fiber loss and for the implementation of long-haul transmission. To achieve a larger span between intermediate repeaters while maintaining an appropriate signal to noise ratio at a receiving end requires an increase in optical power supplied to an optical fiber. The increase in optical power supplied to an optical fiber, however, results in an increase in the nonlinear optical effects of the optical fiber on the transmission characteristics. Especially in dense wavelength division multiplexing transmissions, the effect of XPM (Cross Phase Modulation) becomes a problem. XPM causes degradation in the waveforms of optical signals with different wavelengths propagating through an optical fiber, due to the interaction of the signals traveling through the optical fiber simultaneously for a short period of time although at different group velocities provided by dispersion.
Much research on XPM-caused waveform degradation has been conducted. For example, a report by M. Suyama et al. shows that RZ (Return-to-Zero) codes are less degraded by XPM than NRZ (Non-Return-to-Zero) codes (Optical Fiber Communication (OFC) 1996, Technical Document PD26-2). As demonstrated in this report, it has been found that RZ codes are suitable for long-haul transmission using optical fiber amplifiers in high-speed/high-density wavelength division multiplexing optical transmission systems.
In this context, an optical modulator including an NRZ/RZ converter for subjecting data signals and clock signals to NRZ/RZ conversion to provide RZ-coded output signals and a drive circuit for driving an optical intensity modulator with the RZ-coded output signals as drive signals are used (Japanese Patent Laid-Open Publication No. 2001-147411). In this optical modulator, the optical intensity modulator modulates light received from an optical source with an RZ-coded drive signal to provide an RZ-coded optical signal. Such a conventional optical modulator can provide an output of RZ-coded optical signals and is an effective solution to the XPM problem.
In the conventional optical modulator, however, in operating with RZ codes, the optical intensity modulator, NRZ/RZ converter and drive circuit are required to have a large frequency bandwidth. Such a wideband optical intensity modulator requires a modulation voltage higher than that of a narrowband optical intensity modulator for operation. Further, the drive circuit for driving the wideband optical intensity modulator needs to operate at high speeds with a larger output amplitude value than that of a drive circuit for driving a narrowband optical intensity modulator. In short, a drive circuit with high voltage output is required. However, high-speed drive circuits tend to use smaller-sized transistors and have lower breakdown voltage. This results in technical difficulties in configuring a high-speed high-voltage-output drive circuit.
Thus an apparatus and a method for optical modulation for outputting RZ-coded optical signals as optical transmission signals to suppress degradation due to XPM, eliminating the need for an optical modulator to have a large frequency bandwidth, is required.
An object of the present invention is to provide an apparatus for optical modulation and a method of optical modulation which enable the output of an RZ-coded optical signal as an optical transmission signal to suppress degradation due to XPM and eliminate the need for an optical modulator to have a large frequency bandwidth.
An optical modulator according to an aspect of the present invention comprises a phase modulator configured to drive light with an NRZI-coded drive signal for phase modulation and an intensity modulator to modulate the intensity of a phase-modulated NRZI-coded optical signal received from the phase modulator. The operating point of the intensity modulator is set such that the output of the intensity-modulated optical signal is eliminated when the NRZI-coded optical signal is not varied.
An NRZI (Non-Return-to-Zero-Inverted) code is a code generated in a feedback shift register represented by generating polynomial X+1. An RZ (return-to-zero) code is a code of a type sending pulses shorter than the bit time length of the code and returning to zero during the remaining time without sending any pulse.
In this optical modulator, the phase modulator drives light with an NRZI-coded drive signal to generate an NRZI-coded optical signal which is supplied to the intensity modulator. The intensity modulator is prevented from outputting an intensity-modulated optical signal when the received phase-modulated NRZI-coded optical signal is not varied so that an optical signal output by the intensity modulator is an RZ-coded optical signal. The intensity modulator can thus output an RZ-coded optical signal as an optical signal sent out to an optical transmission path, preventing degradation due to XPM. The phase modulator is driven by an NRZI-coded drive signal, so that a frequency bandwidth required for the phase modulator is only approximately half the bit rate of a data signal by Nyquist theorem. The phase modulator thus does not need to have a large frequency bandwidth.